add vendoring with go dep

vendor/golang.org/x/net/ipv6/control_windows.go

@@ -0,0 +1,16 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipv6
+
+import (
+	"syscall"
+
+	"golang.org/x/net/internal/socket"
+)
+
+func setControlMessage(c *socket.Conn, opt *rawOpt, cf ControlFlags, on bool) error {
+	// TODO(mikio): implement this
+	return syscall.EWINDOWS
+}